Method for detecting an unsealed location in a heat recovery system

ABSTRACT

The invention relates to a method for detecting an unsealed location in a heat recovery system of an internal combustion engine of a motor vehicle. The heat recovery system has at least one working medium, in particular a combustible working medium, and a working medium circuit with at least one evaporator, a pump, and at least one expansion machine to allow an early and reliable detection of leakages in the evaporator.

The invention relates to a method for detecting a leaking point in a heat recovery system of an internal combustion engine of a motor vehicle, wherein the heat recovery system has at least one working medium, in particular a combustible working medium, and a working medium circuit having at least one evaporator, a pump and at least one expansion machine, wherein the exhaust gas of the internal combustion engine flows through the evaporator. Furthermore, the invention relates to an internal combustion engine with a heat recovery system which has at least one, in particular combustible, working medium and a working medium circuit with at least one evaporator, a pump and at least one expansion machine, wherein the evaporator is arranged downstream of an SCR catalytic converter in the exhaust gas flow path of the internal combustion engine, for carrying out this method.

When operating a system for heat recovery with a combustible working medium in conjunction with an internal combustion engine and an evaporator, in particular an EGR evaporator (EGR evaporator: EGR=Exhaust Gas Recirculation), the detection of leakages in the system is of high priority. Leakages in a heat recovery system can lead to the following critical scenarios, among others:

-   -   Outlet of the working medium into the environment—leads to fire         hazard when using a flammable working medium such as ethanol.     -   Entry of the combustible working medium into the internal         combustion engine—causes damage if, for example, the working         medium enters the combustion chamber via an EGR evaporator.     -   Overheating of system components due to insufficient filling         level of the working medium—can, for example, lead to         overheating of the exhaust gas evaporator if the working medium         mass flow rate is too low.

The following methods, for example, are known to detect a leakage in a heat recovery system:

-   -   Monitoring of the filling level of the working medium in the         expansion tank by means of a filling level sensor. If the         filling level is too low, a leakage is indicated.     -   Leak test by pressurizing the deactivated cold system and then         observing the pressure gradient. A too rapid drop in pressure         indicates a leakage.     -   Measuring the electrical conductivity of the insulation of the         heat recovery system. A change in conductivity is a sign of         leakage.

U.S. Pat No. 6,526,358 B1, for example, describes a method for detecting leaks and blockages in a fluid circuit, in which pressure, temperature and flow rate are measured and correlated at various points in the circuit.

JP 2010-156314 A discloses a heat recovery system for an internal combustion engine, wherein O₂ sensors are arranged in the coolant circuit of the heat recovery system for leakage detection.

Known methods have the disadvantage that they can only be carried out when the vehicle is deactivated and/or that devices such as additional sensors are required.

It is the object of the invention to detect leakages in the evaporator of a heat recovery system early and reliably in the simplest possible way.

In accordance with the invention, this is achieved by arranging at least one NH₃ sensor in the exhaust gas flow path downstream of the evaporator and measuring the exhaust gas—preferably in normal operation of the internal combustion engine—with the aid of the NH₃ sensor, wherein if at least one abnormally high NH₃ measured value occurs—preferably after a plausibility check has been carried out—a leakage of the evaporator is concluded. The NH₃ sensor which is already standard in an SCR catalytic converter is thus placed at a point in the exhaust gas flow path downstream of a possible leakage of the evaporator of the heat recovery system. Leakage is understood to be a defect in tightness, wherein the working medium escapes uncontrolled from the evaporator.

A prerequisite for a statement on the tightness of the heat recovery system is that the NH₃ sensor responds to a working medium escaping from the evaporator. To achieve this, a chemical substance is selected as the working medium or a chemical substance is added to the working medium to which the NH₃ sensor reacts, wherein the NH₃ sensor is preferably cross-sensitive with respect to this substance. In general, cross-sensitivity is generally understood as being the sensitivity of a sensor to quantities other than the quantity to be measured, which can lead to deterioration in the measurement result. In the present case, cross-sensitivity of the NH₃ sensor means that it is sensitive to chemical substances other than ammonia. For example, it is known that NH₃ sensors have a cross-sensitivity to NO_(x), especially NO₂.

Surprisingly, NH₃ sensors have also been shown to be clearly cross-sensitive to other chemical substances such as ethanol, which is usually used as a working medium. The present invention makes use of the fact that NH₃ sensors not only react to ammonia, but also to ethanol, for example, in a preferred embodiment variant of the invention. Thus, a leakage of the heat recovery system can be inferred if an abnormally high NH₃ measured value is detected. Abnormal in this context means that the detected NH₃ measured value is higher than a value that would be permitted by the instantaneous operating point. In order to verify or falsify this, a plausibility check is advantageously carried out with regard to the NH₃ measurement value determined. If the evaporator is located downstream of an SCR catalyst, the plausibility check includes whether the increased NH₃ reading can be attributed to the injection of an NH₃-containing additive.

An SCR catalytic converter (SCR=Selective Catalytic Reduction) in the exhaust gas line of an internal combustion engine serves to reduce nitrogen oxides occurring during combustion. For example, NH₃ is required for the reaction, which is added to the exhaust gas upstream of the SCR catalyst in the form of an aqueous urea solution. The products of the reaction are H₂O (water) and N₂ (nitrogen). To control the exact dosing of the urea solution, at least one NH₃ sensor is usually installed in the exhaust gas line downstream of the SCR catalytic converter. In the context of the present invention, this NH₃ sensor is not arranged—as usual—directly after the SCR catalyst or an ammonia barrier catalyst (ASC) possibly downstream of the SCR catalyst, but downstream of an evaporator of the waste heat recovery system arranged after the SCR catalyst or ammonia barrier catalyst. The barrier catalyst is used to reduce or remove optionally excessive NH₃ downstream of the SCR storage catalyst 8, wherein ammonia is oxidized to N₂ and H₂O.

In order to be able to exclude influences of the exhaust aftertreatment on the increased NH₃ measured value, a plausibility check must be carried out in normal engine operation of the internal combustion engine if an abnormally high measured value occurs, in accordance with an advantageous embodiment variant, by reducing or stopping the injection quantity of an NH₃-containing additive upstream of the SCR storage catalyst—preferably using a closed control circuit—and examining whether, after a defined first waiting time has elapsed, a substantial reduction in the measured NH₃ concentration occurs, and, if an abnormally high NH₃ measured value is further determined, a leakage of the evaporator is concluded.

As an alternative to or in addition to the leakage investigation in engine operation, it can also be provided in accordance with an advantageous embodiment of the invention that, in the motor-braking operation of the internal combustion engine, the injection of an NH₃-containing additive is reduced or stopped upstream of the SCR storage catalytic converter, and it is investigated whether, after a defined second waiting time has elapsed, a substantial reduction in the measured NH₃ concentration occurs, and, if an abnormally high NH₃ measured value is further determined, a leakage of the evaporator is concluded.

For example, the first and/or second waiting time should be at least 30 to 60 seconds in order to achieve a reaction equilibrium state.

Alternatively or in addition to an evaporator in the exhaust gas line downstream of the SCR catalytic converter, a (further) evaporator of the waste heat recovery system can be arranged in an exhaust gas recirculation line. Here, in most cases, if an abnormally elevated NH₃ reading is detected, causes other than evaporator leakage can be excluded.

A leakage of the second evaporator can also be detected—for example in motor-braking operation—by the first NH₃ sensor in the exhaust gas line downstream of the first evaporator, provided that the working medium of the working medium circuit escaping at the leakage point of the second evaporator does not enter into a chemical reaction within the combustion chambers or in the exhaust aftertreatment system whose end product can no longer be detected by the NH₃ sensor. In order to exclude this risk, it may be provided, in a further development of the invention, that a second NH₃ sensor is arranged downstream of the second evaporator in the exhaust gas recirculation line, via which leakages of the second evaporator can be detected directly.

For example, an abnormally high NH₃ reading is present if the average NH₃ reading is above 10 ppm.

If there is no doubt that the evaporator is leaking, a respective warning signal can be issued to the driver and/or a corresponding entry made in the error code in the on-board diagnostic system.

The invention is explained in detail below using a non-restrictive embodiment example shown in the drawings, wherein:

FIG. 1a schematically shows an internal combustion engine according to the invention in a first embodiment variant;

FIG. 1b schematically shows an internal combustion engine according to the invention in a second embodiment variant;

FIG. 2 and FIG. 3 show a time curve of the load and the NH₃ measured values for a leakage-free heat recovery system; and

FIG. 4 and FIG. 5 show a time curve of the load and the NH₃ measured values for a leaky heat recovery system when applying the present invention.

FIG. 1a and FIG. 1b each show an internal combustion engine 1 with exhaust gas flow paths 2, which are formed by an exhaust gas line 3 and an exhaust gas recirculation line 4. The exhaust gas recirculation line 4 is used for external exhaust gas recirculation between exhaust system 22 and intake system 23 of the combustion engine 1.

An exhaust aftertreatment device 5 is arranged in the exhaust gas line 3, which has a diesel oxidation catalytic converter 6, a diesel particulate filter 7, an SCR catalytic converter 8 and a barrier catalytic converter 9 in the embodiment example. An NH₃-containing additive can be fed upstream of the SCR catalyst 8 via an injection device 10. A mixer 11 is used to mix and evaporate the injected additive in the exhaust gas stream.

A heat recovery system 12 with a closed working medium circuit 13 for a working medium, for example ethanol, which has at least one first evaporator 14, a pump 15 and at least one expansion machine 16, is provided for recovering the exhaust gas waste heat. Reference numeral 17 denotes a condenser. To bypass the first evaporator 14 on the exhaust side, a bypass line 18 is provided, which branches off from the exhaust gas line 3 upstream of the first evaporator 14 and flows back into the exhaust gas line 3 downstream of the first evaporator 14. Reference numeral 19 denotes a control element formed, for example, by a changeover flap for switching the exhaust gas flow between the flow path through the first evaporator 14 and the bypass line 18.

Optionally, a second evaporator 14 a may be provided in the exhaust gas recirculation line 4, which may be integrated upstream or downstream of the first evaporator 14 into the working medium circuit 13 of the heat recovery system 12.

In the first embodiment variant shown in FIG. 1 a, the second evaporator 14 a is integrated downstream of the first evaporator 14 into the working medium circuit 13. Alternatively, the second evaporator 14 a can also be arranged in a second working medium circuit 13 a of the heat recovery system 12 comprising a second pump 15 a, a second expansion machine 16 a and a second condenser 17 a, as shown in FIG. 1 b.

Downstream of the first evaporator 14, an NH₃ sensor 20 is arranged in the exhaust gas line 3. Another NH₃ sensor 20 a may be located downstream of the second evaporator 20 a in the exhaust gas recirculation line 4. The NH₃ sensors 20, 20 a are connected to a control and/or evaluation unit 21.

The NH₃ sensors 20, 20 a each have a cross-sensitivity to the working medium—in this case to ethanol—or to a component of the working medium. This means that the NH₃ sensors not only detect NH₃ measured values from ammonia actually contained in the exhaust gas, but also from the substance initiating the cross-sensitivity. If there is a leakage in the area of evaporator 14 or 14 a, this is reflected in the output increased NH₃ measured values of the NH₃ sensors 20 or 20 a.

If an abnormally high NH₃ measured value of the first NH₃ sensor 20 occurs during normal engine operation of internal combustion engine 1, this NH₃ measurement value is subjected to a plausibility check, in that the injection quantity of the NH₃-containing additive is reduced or stopped by the injection device 10 upstream of the SCR storage catalyst 8—for example using a closed control loop—by means of the injection device 10, and it is investigated whether a substantial reduction in the measured NH₃ concentration occurs after a defined first waiting time (for example 30 to 60 seconds) has elapsed. If an abnormally high NH₃ reading continues to be detected, a leakage at the first evaporator 14 may be concluded.

As an alternative to engine operation, the method according to the invention for detecting an unsealed location in the heat recovery system 12 can also be carried out during motor-braking operation or when switching from engine operation to motor-braking operation. During motor-braking operation of internal combustion engine 1, the injection of an NH₃-containing additive upstream of the SCR storage catalyst 8 is stopped and it is investigated whether a significant reduction in the measured NH₃ concentration occurs after a defined second waiting time of 30 to 60 seconds, for example. If there is no significant reduction in the NH₃ measured value, a leakage at the first evaporator 14 can be concluded. If it cannot be ruled out that a working medium possibly escaping from the second evaporator 14 a is eliminated or chemically modified in the internal combustion engine 1 or in the exhaust aftertreatment system 5, it is advantageous to use a second NH₃ sensor 20 a directly downstream of the second evaporator 14 a in the exhaust gas recirculation line 4, via which leakages of the second evaporator 14 a can be detected directly.

FIG. 2 and FIG. 3 show a time curve of the load L and the NH₃ measured values for a leakage-free heat recovery system 12 during a changeover of the internal combustion engine 1 to motor-braking operation. Usually the injection of the NH₃ additive is stopped during motor-braking operation. It can clearly be seen that the measured NH₃ measured value of the first NH₃ sensor 20 decreases drastically—there is therefore no leakage.

FIG. 4 and FIG. 5, on the other hand, show a changeover to motor-braking operation if there is a leakage in the first evaporator 14. In this case there is no reduction in the NH₃ measured value. On the contrary—due to the unchanged leakage and the lower gas throughput due to the exhaust gas path in motor-braking operation, there may even be an increase in the NH₃ measured value, as the dashed line shows. In this case, a leakage of the evaporator 14 can be clearly identified and a corresponding leakage warning can be issued to the driver.

The main advantage of this invention is that it does not require the use of overly complex leakage detectors. Ideally, an NH₃ sensor 14 already fitted as standard in the exhaust gas line—albeit at a point downstream of the evaporator in the exhaust gas flow path 2—can be used to carry out reliable leakage tests. 

1. A method for detecting a leak in a heat recovery system of an internal combustion engine of a motor vehicle, wherein the heat recovery system comprises: at least one particularly combustible working medium; a working medium circuit having at least one evaporator, arranged in an exhaust gas flow path of the internal combustion engine; a pump; at least one expansion machine; wherein at least one NH₃ sensor is arranged downstream of the at least one evaporator in the exhaust gas flow path, and configured to measure the exhaust gas and wherein after the occurrence of at least one abnormally high NH₃ measured value a conclusion is drawn on a leakage of the evaporator.
 2. The method according to claim 1, wherein a chemical substance serves as a working medium to which the NH₃ sensor reacts.
 3. The method according to claim 1, wherein a plausibility check is carried out in the engine operation of the internal combustion engine when an abnormally high NH₃ measured value is detected, in that the injection quantity of an NH₃-containing additive is reduced or stopped upstream of the SCR storage catalytic converter, and checking whether after the expiration of a defined first waiting time, a substantial reduction in the measured NH₃ concentration occurs, and if an abnormally high NH₃ measured value is still detected, a conclusion is drawn on a leakage of the evaporator.
 4. The method according to claim 1, wherein the injection of an NH₃-containing additive upstream of the SCR storage catalytic converter is reduced or stopped during the motor-braking operation of the internal combustion engine, and it is examined whether, after the expiration of a defined second waiting period, a substantial reduction in the measured NH₃ concentration occurs, and if an abnormally high NH₃ measured value is still detected, a conclusion is drawn on a leakage of the evaporator.
 5. The method according to claim 1, to wherein an abnormally high NH₃ measured value is determined when the average NH₃ measured value is above 10 ppm.
 6. The method according to claim 1, wherein the first and/or second waiting time is at least 30 to 60 seconds.
 7. The method according to claim 1, wherein a corresponding warning signal is output when a leakage of the evaporator is detected.
 8. A heat recovery system of an internal combustion engine, the heat recovery system comprising: at least one particularly combustible working medium; a working medium circuit with at least one evaporator arranged in an exhaust gas flow path of the internal combustion engine; a pump; at least one expansion machine; and at least one NH₃ sensor arranged downstream of the at least one evaporator and in the exhaust gas flow path, and configured to measure the exhaust gas, and wherein the NH₃ sensor is communicatively coupled to an electronic control and/or evaluation unit.
 9. The heat recovery system according to claim 8, wherein a first evaporator of the at least one evaporator is arranged downstream of an SCR catalytic converter in an exhaust gas line of the internal combustion engine forming the exhaust gas flow path, wherein a first NH₃ sensor is arranged downstream of the first evaporator in the exhaust gas line.
 10. The heat recovery system according to claim 9, wherein the first evaporator is configured to be selectively bypassed by a bypass line which branches off from the exhaust gas line upstream of the first evaporator and enters the exhaust gas line again downstream of the first evaporator.
 11. The heat recovery system according to claim 8, wherein a second evaporator is arranged in an exhaust gas return line of the internal combustion engine forming the exhaust gas flow path.
 12. The method according to claim 1, wherein a plausibility check is carried out on the measured value before the conclusion is drawn on the leakage of the evaporator.
 13. The method according to claim 2, wherein the NH₃ sensor has a cross-sensitivity to said chemical substance.
 14. The method according to claim 3, wherein the injection quantity of an NH₃-containing additive is reduced or stopped upstream of the SCR storage catalytic converter by using a closed control loop.
 15. The internal combustion engine according to claim 9, wherein a barrier catalyst for NH₃ is arranged between the SCR catalytic converter and the first evaporator.
 16. The internal combustion engine according to claim 9, wherein the first NH₃ sensor is arranged downstream of the orifice of the bypass line into the exhaust gas line.
 17. The internal combustion engine according to claim 11 wherein a second NH₃ sensor is arranged downstream of the second evaporator in the exhaust gas return line. 